Mobile device form factors continue to trend smaller and thinner even while the number of options for wireless connectivity and communication using such devices increases. A typical mobile device today supports a variety of such options (e.g., GSM (2G), 3G, 4G, Wi-Fi, Bluetooth, etc.) and must be able to transmit and receive RF energy in many different transmission bands over a wide frequency range (e.g., 700 MHz to 3 GHz); often in more than one band simultaneously. It is a difficult engineering challenge to design an antenna that can transmit and receive RF energy efficiently in so many bands over such a broad frequency range; particularly given the limits on out-of-band harmonic energy specified by FCC regulations. Such harmonic energy corresponds to component frequencies of the RF signal that are integer multiples of the fundamental frequency of the RF signal.
One approach to attenuating this harmonic energy to bring it within acceptable limits is to employ hardware filters (e.g., low-pass or notch filters) to attenuate undesirable out-of-band signal components. However, such hardware solutions negatively impact design size, cost and complexity (e.g., because of the additional hardware components required), as well as have undesirable impacts on other transmission bands (e.g., a filter designed to attenuate energy at a harmonic frequency associated with one band might attenuate energy at the fundamental frequency in another). Another approach is to reduce the overall transmit power under certain conditions for a given transmission band to ensure that related harmonics are within acceptable levels. However, this approach does not take full advantage of the transmission efficiency for which device antennas are typically designed, and can result in performance problems such as, for example, dropped calls and low data throughput.